The
Vapor-Liquid-Solid (VLS) method is one of the most popular techniques for
growing semiconducting nanowires, and the stability of the liquid droplet is an
important factor controlling wire morphology and, ultimately, functionality.
Earlier theoretical work on axisymmetric systems indicates that the
lowest-energy liquid configuration varies with surface energies, wire radius,
and fluid volume. We test these predictions with a fully dynamic phase-field
model that incorporates viscous fluid flow. Under conditions predicted by
earlier theoretical work, we observe the pinning of the liquid to the top face
of a nanowire, a condition necessary for wire growth. To study the stability of
the droplet, we apply perturbations to the liquid shape and find that the
system can transition to a metastable configuration, a local minimum in the
energy landscape. Furthermore, the transition pathway to this local minimum
depends on the magnitude of the perturbations. Under conditions that favor a
liquid on the sidewalls of the wire, we observe a spontaneous transition of the
liquid from a droplet to an annular configuration through an intermediate state
that is not predicted by theory. The time scales and contact-line speeds for
these transitions are determined through simulation and are consistent with
approximations based on simple dimensional analysis.